1. Field of the Invention
The present invention relates to a standard zoom lens system for use in a small and light-weight digital compact camera, in particular, relates to a standard zoom lens system having the following features:
(i) telecentricity is attained;
(ii) the number of lens elements is small;
(iii) low production cost is attained;
(iv) an angle-of-view is approximately from 22° to 62°; and
(v) a zoom ratio is approximately 3.
2. Description of the Prior Art
In recent years, further miniaturization of digital compact cameras have progressed due to further miniaturization of electronic components; and thereby, further miniaturization of the photographing optical system is also required.
On the other hand, due to further progress in higher pixelization of imaging devices, the photographing optical systems are required to have higher resolution. Moreover, in order to prevent shading and color shift, excellent telecentricity by which light emitting from the most image-side lens surface is orthogonally incident on the imaging surface is required in the photographing optical system of a digital camera.
As a standard zoom lens system for use in a compact digital camera, a negative-lead type lens system is often employed in the case of a zoom ratio up to approximately 3.
A negative-lead type lens system can achieve a wider angle-of-view at the short focal length extremity, and can have smaller lens diameters, especially the diameter of the most object-side lens group can be made smaller. Therefore a negative-lead type lens system is suitable for a retractable zoom lens system in which the distance between each lens group is reduced as the lens groups retract, in an optical axis direction thereof, to an accommodation position.
Furthermore, since the exit pupil needs to be positioned sufficiently far from the imaging plane, a negative-lead type lens system of a three-lens-group arrangement, i.e., a lens group having a negative refractive power (hereinafter, a negative lens group), a lens group having a positive refractive power (hereinafter, a positive lens group) and a positive lens group in this order from the object, is often used.
Examples of the prior art can be found in Japanese Unexamined Patent Publication Nos. 2002-277740, 2002-90624 and 2003-140041.
Japanese Unexamined Patent Publication No. 2002-277740 discloses zoom lens systems having a zoom ratio of approximately 2.4 to 3; however, it is understood from this publication that a zoom lens system with a larger zoom ratio, the entire length thereof becomes longer accordingly. Therefore sufficient miniaturization of the zoom lens system cannot be achieved.
Japanese Unexamined Patent Publication No. 2002-90624 discloses a zoom lens system of a four-lens-groups arrangement with a relatively large number of lens elements, so that such a zoom lens system does not satisfy the requirement of low production costs. Furthermore, the diameter of the most object-side lens group and the overall length of the zoom lens system are large with respect to the focal length, so that miniaturization thereof is insufficient.
Japanese Unexamined Patent Publication No. 2003-140041 discloses a relatively inexpensive optical system having a reduced number of lens elements; however, the thickness of each lens group is large. As a result, the length of the zoom lens system in the fully-retracted state becomes longer, which increases the size of the zoom lens system.
The miniaturization of a retractable standard zoom lens system requires both a reduction in the overall length of the zoom lens system and a reduction in the thickness of each lens group. Generally, the reduction of the number of lens elements, in order to miniaturize the zoom lens system and reduce the thickness of the lens groups, increases difficulties in correcting aberrations. In order to achieve miniaturization while adequately correcting various aberrations over the entire zooming range, appropriate refractive power distribution over each lens group and an appropriate lens arrangement are required.